The present invention relates to adsorber vessels for use in the adsorption of components from gases and subsequent regeneration of the adsorbent by thermally induced desorption of adsorbed components. It further relates to apparatus for use in the removal of components from gas mixtures by swing adsorption including temperature swing adsorption and related techniques and thermally assisted pressure swing adsorption as well as methods for the operation of such apparatus, including the pre-purification of air prior to cryogenic distillation.
It is necessary in a number of circumstances to remove gas components from a gas stream by adsorption on a solid adsorbent, with periodic regeneration of the adsorbent. The removed gas components may be of intrinsic value or they may be contaminating components in the gas mixture.
In such methods the gas stream is conventionally fed in contact with a solid adsorbent contained in an absorber vessel to adsorb the component or components to be removed and these gradually build-up in the adsorbent. The concentration of the removed component or components in the adsorbent gradually rises and if the process is continued for a sufficient period, the adsorbed components will break through the downstream end of the adsorbent bed. Before this occurs, it is necessary to regenerate the adsorbent.
In a pressure swing adsorption (PSA) system, this is done by stopping the flow into the adsorbent of gas to be treated, depressurising the adsorbent and, usually, by passing a flow of regenerating gas low in its content of the component adsorbed on the bed through the bed counter-current to the product feed direction. Optionally, some heat may be added to the regenerating gas flow but normally the aim is to commence regeneration before the heat produced by adsorption of the adsorbed component on the adsorbent bed has progressed out of the adsorbent containing vessel.
The direction of the heat pulse is reversed by the process of regeneration and the heat which derived from the adsorption of the gas component in question is used for desorbing that component during regeneration. This at least largely avoids the need to add heat during the regeneration step.
An alternative procedure is known as temperature swing adsorption (TSA). In TSA, the cycle time is extended and the heat pulse mentioned above is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-line period. To achieve regeneration it is therefore necessary to supply heat to desorb the adsorbed gas component. To this end, the regenerating gas used is heated for a period to produce a heat pulse moving through the bed counter-current to the normal feed direction. This flow of heated regenerating gas is usually followed by a flow of cool regenerating gas which continues the displacement of the heat pulse through the bed towards the upstream end. TSA is characterised by an extended cycle time as compared to PSA. A variant of TSA is described by von Gemmingen, U. in “Designs of Adsorptive Driers in Air Separation Plants”—Reports on Technology 54–94—(Linde) using lower than normal temperatures, i.e. 80°to 130° C. and short cycle times.
A modification of the classical TSA process which is known as TPSA is described in U.S. Pat. No. 5,855,650. Here, the regenerating gas is heated during a period of regeneration such that the heat added to the regenerating gas is no more than 90 percent of the heat of adsorption liberated during the adsorption of the gas components which are adsorbed. Regeneration is continued after the ending of heating of the regenerating gas to continue to desorb the adsorbed gas stream component.
A further variant on the TSA process known as TEPSA is described in U.S. Pat. No. 5,614,000. Here, two different gas components such as water and carbon dioxide are adsorbed during the on-line period. A heated regenerating gas is fed during regeneration counter-current to the feed direction to produce a heat pulse travelling in the counter-current direction to desorb the less strongly adsorbed of the two adsorbed components. Heating of the regenerating gas is then terminated and feeding of the regenerating gas continues to allow the more strongly adsorbed component to be desorbed by pressure swing desorption, the regenerating gas being fed at a pressure lower than the pressure during the on-line period.
Whilst the present invention is of relevance to classical TSA systems, it is particularly suitable for use with TPSA or TEPSA systems where usually the period for which the regenerating gas is heated is reduced in comparison to classical TSA systems.
Usually in systems of the kind described above, at least two adsorber vessels are present and are connected in parallel. At any given time, one of these is on-line to adsorb one or more components from a feed gas stream whilst the other is being regenerated or is waiting for regeneration or to come back on-line. The adsorbent containing vessels are each connected through a first manifold to a source of gas which is to be purified. The downstream ends of the adsorbent vessels are similarly connected via a second manifold to a source of regenerating gas.
The usual arrangement has been for a heater for the regenerating gas to be provided at a location which is down-stream of the second manifold in the product feed direction (upstream in the regenerating gas feed direction) so that it serves to provide heated gas to either of the two adsorber vessels as required.
An alternative arrangement is to provide a heater inside each of the adsorber vessels upstream of or within the adsorbent (see U.S. Pat. No. 5,213,593, WO 96/14917 and U.S. Pat. No. 3,193,985 as typical examples). Such a heater may extend through an internal pipe running from one end of the vessel to close to the other end.
As we have now appreciated, both of these arrangements have significant disadvantages. Where a common heater is positioned on the other side of a manifold connecting to both adsorber vessels, it takes a considerable period of time for gas heated in the heater to reach and begin to heat the adsorbent in the beds. Allowance must be made not only for the time required for the gas to be passed from the heater to the adsorbent beds but also for the provision of extra heat due to the need for the gas to heat up all of the intervening pipe and valve work before the full temperature of the heated regenerating gas begins to be applied to the adsorbent. This is of particular concern where the period during which the regeneration gas is heated is relatively short as in TPSA and TEPSA type systems.
On the other hand, where the heater is installed in the adsorbent containing vessel itself this gives rise to difficulties in access for heater maintenance and difficulties in filling the adsorber vessels with adsorbent. It also gives rise to difficulty due to lack of mixing, maldistribution and less than optimum heat transfer in ensuring that each particle of adsorbent is heated to an adequate temperature for full regeneration. Adsorbent materials are not characterised by high thermal conductivity and the heat transfer coefficient between the gas and the adsorbent is poor. Various types of heater have been used inside adsorber vessels including microwave radiation, electric heater elements, external heating jackets with radial fins protruding into the vessel interior and the use of axially extending Curie point heaters. However, all of these direct heating methods suffer from the difficulties discussed above.
EP 1072302 A discloses an arrangement in which a tube having catalytic material in porous walls depends from the body of a chamber containing a heater. Regeneration gas is passed into the chamber to heat and flow through the catalytic material.